The present invention relates to a logic circuit with Josephson junctions, and more particularly to a logic circuit with at least one asymmetrical quantum interferometric circuit having asymmetrical bias curent vs. control current characteristics.
Logic gates in high-impedance circuits have heretofore been comprised of a symmetrical quantum interferometric circuit such as shown in "Applied Physics Letters", 25, No. 7, pp. 426-428(1974) entitled "Experimental observation of the switching transients resulting from single flux quantum transitions in superconducting Josephson devices" by P. Gueret. In such a symmetrical quantum interferometric circuit, a Josephson junction J.sub.1 having a threshold current I.sub.j1 is connected in series with an inductance element having an inductance L.sub.1, and a Josephson junction J.sub.2 having a threshold current I.sub.j2 is connected in series with an inductance element having an inductance L.sub.2. Such two series-connected circuits are connected in parallel with each other and are supplied with a bias current I.sub.B. A control current I.sub.C flows through two series-connected inductance elements which are electromagnetically coupled with the inductances L.sub.1, L.sub.2, respectively. At this time, the following relations are established: EQU I.sub.j1 =I.sub.j2, L.sub.1 =L.sub.2, and L.sub.1 I.sub.j1 =L.sub.2 I.sub.j2 &lt;.PHI..sub.o.
where .PHI..sub.o is a flux quantum unit equal to 2.times.10.sup.-15 Wb.
The voltage state of such a symmetrical quantum interferometric circuit is dependent on the magnetic field generated by the control current I.sub.C. More specifically, an I.sub.B -I.sub.C characteristic has a threshold curve that is symmetrical with respect to the axis of the bias current I.sub.B, and the zero voltage state takes place within an area that is determined by the threshold value of I.sub.B and the threshold curve while a finite voltage takes place outside such an area. However, such a threshold curve is spread in the direction of the horizontal axis which scales the control current I.sub.C and its slope is 1(one) at the most. A control current of a relatively large amplitude and hence a great amount of power consumption are required for causing the apparatus to switch between the zero and finite voltages.
One approach toward amplification of the slope of the threshold curve to more than 1 is to use an asymmetric quantum interferometric circuit as described in "Asymmetric 2-Josephson-junction Interferometer as a Logic Gate" by H. Beha, Electronics Letters, 31st Mar. 1977, vol. 13, No. 7. With such a device, the values of threshold currents and inductances are asymmetrical, that is, I.sub.j1 .noteq.I.sub.j2, and L.sub.1 .noteq.L.sub.2. This prior art device, however, has a small range of bias current available for the switching between zero and finite voltages, and thus covers only a limited range of operation. Furthermore, it is difficult to reduce the amplitude of the control current. Accordingly, such a device has practically been infeasible.